The invention relates to generators for portable devices, and more particularly to piezoelectric generators.
Transducers such as piezoelectrics, electrostrictors, and magnetostrictors, can be used to convert one form of energy to another. Energy from a mechanical input, for example, a periodic force applied to a device containing a piezoelectric or electrostrictive material, can be converted to electric energy. Therefore, such materials provide a means for harvesting electric power from a mechanical input.
The equations for such a piezoelectric element can be written as:
kyxe2x88x92Nv=F 
Ny+Cv=Q 
where y is the deformation of the transducer element, F is the force applied, v is the voltage across the electrodes of the transducer, Q is the charge produced, k is the equivalent stiffness of the transducer taking into account any mechanical amplification or geometric factors, N is the piezoelectric constant scaled by appropriate geometric factors, and C is the capacitance of the device.
A generator employs piezoelectric elements to convert mechanical power to electrical power. The generator includes one or more piezoelectric transducers that are actuated by a mechanical input. The resulting electrical power is stored or used to run an electronic device. The generator is hand or foot operated.
According to one aspect of the invention, a method of extracting electrical energy from mechanical motion includes reusing an elastic portion of energy in a transducer by transferring the elastic portion of energy to another transducer.
According to another aspect of the invention, an apparatus for extracting electrical energy from mechanical motion includes at least two transducers coupled such that an elastic portion of energy in one transducer is transferable to the other transducer.
Embodiments of this aspect of the invention may include one or more of the following features.
The transducers are coupled by a member defining a waved surface, for example, a sinusoidal surface, and each transducer defines a coupler in contact with the waved surface for movement following the waved surface. The coupler contacts the waved surface on a first side of the coupler. The member defines a second waved surface, and the coupler contacts the second waved surface on a second side of the coupler opposite the first side. Couplers of two transducers are positioned such that they move out-of-phase relative to each other.
In a particular embodiment, the transducers are bound to a plate. The plate is positioned between members such that the plate is deformed. The plate and members are configured such that relative rotation therebetween produces a wave that travels along the plate.